Level shifter

ABSTRACT

Input transistors have sources which are connected to a first input reference node and gates to which a pair of input signals are input. Input-side voltage relaxing transistors have sources connected to drains of the pair of input transistors and gates connected to a second input reference node. Output-side voltage relaxing transistors have sources connected to output nodes, gates connected to a first output reference node, and drains connected to drains of the input-side voltage relaxing transistors. First and second inverter circuits are in correspondence with the output nodes, and are connected between second and third output reference nodes. Each of the first and second inverter circuits also supplies a voltage at one of the second and third output reference nodes to its corresponding one of the output nodes, depending on a voltage at its non-corresponding one of the output nodes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a level shifter for shifting a level ofan input signal.

2. Description of the Related Art

In recent years, a level shifter is used in SRAM or flash EEPROMmemories or the like so as to apply a negative voltage to a control gateto drive the memory. Such a level shifter outputs an output signalhaving a varying level which can take a negative value. Such a levelshifter is disclosed in Japanese Unexamined Patent ApplicationPublication No. 11-308092 (Patent Document 1) or the like.

FIG. 8 shows a configuration of the level shifter described in PatentDocument 1. The level shifter outputs an output signal LO having anegative level in response to an input signal IN when a negative voltagecontrol signal C is supplied and an output signal LO having the samelevel as that of the input signal IN when the negative voltage controlsignal C is not supplied. The level shifter comprises a negative voltagegenerating circuit 90 which outputs a voltage having a negative level ora ground level as a negative voltage signal VB1 in response to thenegative voltage control signal C, a pair of transistors (P91, P92)which receive the input signal IN and its inverted input signal INB, apair of transistors (N91, N92) which receive the negative voltage signalVB1, and transistors P93, P94, N93 and N94. The transistors P93, P94,N93 and N94 are used to reduce the source-drain voltage of eachtransistor. When the negative voltage signal VB1 is at the ground level,an inverted control signal CB having the L level is supplied to thetransistors N93 and N94. When the negative voltage signal VB1 is at thenegative level, an inverted control signal CB having the H level issupplied to the transistors N93 and N94. Voltages VA, VB, VC, VD and VEat nodes in the level shifter vary depending on the input signal IN, asshown in FIG. 9. In FIG. 9, “VTP” indicates a threshold voltage of thetransistors P93 and P94, and “VTN” indicates a threshold voltage of thetransistors N93 and N94.

However, in the conventional level shifter, for example, when thecurrent driving ability of the transistor N93 is small, a rise time Tcof the source voltage VC is longer than a rise time Te of the drainvoltage VE in the transistor N93, so that the source-drain voltage islikely to increase. For example, if the source voltage VC has notreached “−VTN” when the drain voltage VE reaches “Vdd”, the maximumvalue of the source-drain voltage of the transistor N93 exceeds“Vdd+VTN”.

Thus, the breakdown voltage of a transistor needs to be strictly limited(e.g., the maximum value of the source-drain voltage of the transistorneeds to be estimated to be large). As a result, the transistor iscaused to have a high breakdown voltage. Therefore, an additionalprocess for increasing the breakdown voltage may disadvantageously leadto an increase in cost or an increase in circuit area.

SUMMARY OF THE INVENTION

An object of the present invention is to reduce the source-drain voltageof a transistor and thereby relax the breakdown voltage limit of thetransistor.

According to the present invention, a level shifter is provided forreceiving a pair of input signals varying with complementary levels andshifting the levels of the pair of input signals, comprising a pair ofinput transistors having a pair of sources connected in common to afirst input reference node and a pair of gates to which the pair ofinput signals are input, a pair of input-side voltage relaxingtransistors having a pair of sources connected to a pair of drains ofthe pair of input transistors and a pair of gates connected in common toa second input reference node, and for limiting voltages at the pair ofdrains of the pair of input transistors, a pair of output nodes, a pairof output-side voltage relaxing transistors having a pair of sourcesconnected to the pair of output nodes, a pair of gates connected incommon to a first output reference node, and a pair of drains connectedto the pair drains of the pair of input-side voltage relaxingtransistors, and for limiting voltages at the pair of output nodes, anda first inverter circuit and a second inverter circuit in one-to-onecorrespondence with the pair of output nodes and each connected betweena second output reference node and a third output reference node. Eachof the first and second inverter circuits supplies a voltage at one ofthe second and third output reference nodes to its corresponding one ofthe output nodes, depending on a voltage at its non-corresponding one ofthe output nodes.

In the level shifter, when a voltage is supplied from the output-sidevoltage relaxing transistor to an output node, a voltage from acorresponding inverter circuit is also supplied to the output node, sothat the rate of variation of a voltage at the output node is increased.Thus, the source-drain voltage can be reduced due to the increasedvoltage variation rate at the output node.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a configuration of a level shifteraccording to a first embodiment of the present invention.

FIG. 2 is a waveform diagram for describing an operation of the levelshifter of FIG. 1.

FIG. 3 is a circuit diagram showing a variation of the level shifter ofFIG. 1.

FIG. 4 is a circuit diagram showing a configuration of a level shifteraccording to a second embodiment of the present invention.

FIG. 5 is a waveform diagram for describing an operation of the levelshifter of FIG. 4.

FIG. 6 is a circuit diagram showing a variation of the level shifter ofFIG. 4.

FIG. 7 is a circuit diagram showing a configuration of a level shifteraccording to a third embodiment of the present invention.

FIG. 8 is a diagram showing a configuration of a conventional levelshifter.

FIG. 9 is a waveform diagram for describing an operation of the levelshifter of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. The same or like parts areindicated by the same reference numerals and will not be repeatedlydescribed.

First Embodiment

FIG. 1 shows a configuration of a level shifter according to a firstembodiment of the present invention. The level shifter shifts the levelsof a pair of input signals (INP, INM) and outputs the results as a pairof output signals (OUTP, OUTM). The level shifter comprises a pair ofinput transistors (101A, 101B), a pair of voltage relaxing transistors(102A, 102B), a pair of voltage relaxing transistors (103A, 103B),inverter circuits 104A and 104B, and output circuits 105A and 105B.

The input signals INP and INM are signals which have complementarylevels varying between a positive level (Vdd) and a ground level (Vss).A reference node N1 (first input reference node) receives a voltage(Vdd) corresponding to the H level of an input signal. A reference nodeN2 (second input reference node) receives a voltage (Vss) correspondingto the L level of an input signal.

A reference node N3 (first output reference node) and a reference nodeN4 (second output reference node) receive a voltage (Vreg) correspondingto the H level of an output signal. A reference node N5 (third outputreference node) receives a voltage (Vssm) corresponding to the L levelof an output signal. Here, the voltage Vssm is a negative voltage. Thereference node N4 and the reference node N3 may be a common node. Notethat the voltages applied to the reference nodes N3 and N4 may not bethe same.

The input transistor pair (101A, 101B) supplies a voltage at thereference node N1 to one of a pair of drains (nodes na1, nb1), dependingon the voltage levels of the input signal pair (INP, INM).

The voltage relaxing transistor pair (102A, 102B) limits voltages (Va1,Vb1) at the nodes na1 and nb1 so that the voltages of the drain pair(nodes na1, nb1) of the input transistor pair (101A, 101B) do not becomelower than “Vss”.

The voltage relaxing transistor pair (103A, 103B) limits voltages (Va3,Vb3) at output nodes N11A and N11B not to exceed “Vreg”.

The inverter circuit 104A corresponds to the output node N11A, and theinverter circuit 104B corresponds to the output node N11B. The invertercircuit 104A supplies one of the output voltages Vreg and Vssm to theoutput node N11A, depending on the voltage at the output node N11B. Theinverter circuit 104A includes select transistors 111P and 111Nconnected in series between the reference nodes N4 and N5. The invertercircuit 104B has a similar configuration.

The output circuits 105A and 105B are auxiliary buffer circuits, have aconfiguration similar to that of the inverter circuits 104A and 104B,and include output transistors 112P and 112N connected in series betweenthe reference nodes N4 and N5. The output circuit 105A outputs one ofthe output voltages Vreg and Vssm as the output signal OUTP, dependingon the voltage at the output node N11A.

Next, an operation of the level shifter of FIG. 1 will be described withreference to FIG. 2. Note that the threshold voltage of the voltagerelaxing transistors 102A and 102B is assumed to be “Vth”. Also, it isassumed that Vdd=2.5 [V], Vs=0 [V], Vreg=1.0 [V], Vssm=−1.5 [V], andVth=0.7 [V]. Specifically, this is an example in which the level shifteris comprised of 2.5-V transistors, and the input signals INP and INMhaving a level varying between “2.5 V” and “0 V” are shifted to theoutput signals OUTP and OUTM having a level varying between “1.0 V” and“−1.5 V”.

Initially, when the input signal INP goes from the “H level (Vdd)” tothe “L level (Vss)”, a current flows through the input transistor 101A,so that the voltage (Va1) at the node na1 starts rising. Since thevoltage relaxing transistor 102A is in the ON state, a current flowsthrough the voltage relaxing transistor 102A, so that a voltage (Va2) ata node na2 starts rising. Since the voltage relaxing transistor 103A isin the ON state, a current flows through the voltage relaxing transistor103A, so that the voltage (Va3) at the output node N11A starts rising.

On the other hand, the input signal INM goes from the “L level (Vss)” tothe “H level (Vdd)”, so that a current of the input transistor 101B isinterrupted, and the voltage (Vb1) at the node nb1 starts dropping.Also, a voltage (Vb2) at a node nb2 starts dropping.

When the voltage (Va3) at the output node N11A starts rising, a currentstarts flowing through the select transistor 111N in the invertercircuit 104B, so that the voltage (Vb3) at the output node N11B startsdropping. Thereby, a current starts flowing through the selecttransistor 111P in the inverter circuit 104A, so that the rising rate ofthe voltage (Va3) at the output node N11A increases. When the voltage(Va3) at the output node N11A reaches the gate voltage (Vreg) of thevoltage relaxing transistor 103A, the current of the voltage relaxingtransistor 103A is interrupted.

When the voltage (Vb3) at the output node N11B becomes lower than thegate voltage (Vreg) of the voltage relaxing transistor 103B, a currentflows through the voltage relaxing transistor 103B, so that the voltage(Vb2) at the node nb2 drops.

Thus, not only a positive-level voltage is supplied from the voltagerelaxing transistor 103A, but also a voltage (positive-level voltage) issupplied from the inverter circuit 104A to the output node N11A, so thatthe voltage at the output node N11A increases at a rate higher than inthe conventional art (i.e., a rise time Tr1 of the voltage Va3 isreduced).

On the other hand, when the input signal INP goes from the “L level(Vss)” to the “H level (Vdd)”, the inverter circuit 104B assists in therising of the voltage at the output node N11B.

As described above, the rate of the variation of a voltage at an outputnode can be increased, so that the variation of the drain voltage of avoltage relaxing transistor (output side) can be allowed to follow thevariation of the source voltage thereof. Thereby, the source-drainvoltage can be reduced, thereby making it possible to relax thebreakdown voltage limit of the voltage relaxing transistor (outputside). Also, the levels of the output signal pair can be quicklytransitioned in response to the transition of the levels of the inputsignal pair, thereby making it possible to improve the response of anoutput signal to an input signal.

Also, a relationship in the magnitude of the current driving abilitybetween each transistor is preferably the following. Note that therelationship in the magnitude of the current driving ability can be setas a ratio (W/L) of a gate width to a gate length.

Input transistor 101A (101B)>Select transistor 111N

Voltage relaxing transistor 102A (102B)>Input transistor 101A (101B)

Voltage relaxing transistor 103A (103B)>Select transistor 111N

By setting the current driving ability of each transistor in thismanner, it is possible to certainly achieve voltage rise/voltage drop ateach node.

Note that the output circuits 105A and 105B may be each connectedbetween the reference node N3 and the reference node N5 instead ofbetween the reference node N4 and the reference node N5. With such anarrangement, it is also possible to output an output signal having alevel varying between the positive level “Vreg” and the negative level“Vssm”.

Also, voltages applied to the reference nodes N1, N2, N3, N4 and N5 arenot limited to Vdd, Vss, Vreg, Vreg and Vssm, and may be set to anyvoltages which satisfy the breakdown voltage limit of each transistor.For example, in FIG. 1, if a voltage applied to the reference nodes N3and N4 is changed from the output voltage Vreg to the ground voltageVss, it is possible to output a pair of output signals (OUTP, OUTM)having a level varying between the ground level “Vss” and the negativelevel “Vssm”.

Variation of First Embodiment

As shown in FIG. 3, if the transistors of FIG. 1 are arranged so thatthe polarities thereof are inverted and voltages Vss (0 [V]), Vdd1 (1[V]), Vdd2 (2 [V]), Vdd2 (2 [V]) and Vdd3 (3 [V]) are applied to thereference nodes N1 to N5, it is possible to shift the input signals INPand INM having a level varying between the positive level “Vdd1” and theground level “Vss” to a pair of output signals (OUTP, OUTM) having alevel varying between the positive level “Vdd3” and the positive level“Vdd2”. Thus, it is possible to output a pair of output signals having alevel varying between a positive level and a positive level.

Second Embodiment

FIG. 4 shows a configuration of a level shifter according to a secondembodiment of the present invention. This level shifter comprises, inaddition to the parts of FIG. 1, auxiliary transistors 201A and 201B andcapacitance elements (capacitance sections) 202A and 202B. The gate ofthe auxiliary transistor 201A is connected to the gate of the inputtransistor 101A (the input signal INP is input thereto), the sourcethereof is connected to the reference node N2, and the drain thereof isconnected to the source of the voltage relaxing transistor 102A. Theauxiliary transistor 201B has a similar configuration. The capacitanceelement 202A is connected between the source and drain of the voltagerelaxing transistor 103A. The capacitance element 202B has a similarconfiguration.

Next, an operation by the auxiliary transistors 201A and 201B of FIG. 4and an operation by the capacitance elements 202A and 202B of FIG. 4will be described with reference to FIG. 5.

[Auxiliary Transistors]

When the input signal INP goes from the “L level (Vss)” to the “H level(Vdd)”, the auxiliary transistor 201A goes to the ON state, so that thesource and gate of the voltage relaxing transistor 102A areshort-circuited. Thereby, the voltage (Va1) at the node na1 drops to“Vss” at a rate higher than in FIG. 1 (i.e., a fall time Tf2 of thevoltage Va1 is reduced).

Also, since the voltage (Va1) at the node na1 goes to the gate voltage“Vss” of the voltage relaxing transistor 102A, the maximum value of thesource-drain voltage of the voltage relaxing transistor 102A goes to“Vss-Vssm”, which is smaller than in FIG. 1.

On the other hand, when the input signal INP goes from the “H level(Vdd)” to the “L level (Vss)”, the auxiliary transistor 201A goes to theOFF state, which is an operation similar to that of FIG. 1.

Also, the auxiliary transistor 201B performs a similar operation.

[Capacitance Elements]

When the input signal INP goes from the “H level (Vdd)” to the “L level(Vss)”, the voltage (Va2) at the node na2 starts rising. Variation ofthe voltage at the node na2 is transferred via the capacitance element202A to the source (output node N11A) of the voltage relaxing transistor103A, so that the voltage (Va3) at the output node N11A starts rising,following the voltage Va2. As a result, the voltage at the output nodeN11A rises at a rate higher than in FIG. 1 (i.e., a rise time Tr3 of thevoltage Va3 is reduced).

On the other hand, when the input signal INP goes from the “L level(Vss)” to the “H level (Vdd)”, the voltage (Va3) at the output node N11Astarts dropping. Variation of the voltage at the output node N11A istransferred via the capacitance element 202A to the drain (node na2) ofthe voltage relaxing transistor 103A, so that the voltage (Va2) at thenode na2 starts dropping, following the voltage Va3. As a result, thevoltage at the node na2 drops at a rate higher than in FIG. 1 (i.e., afall time Tf3 of the voltage Va2 is reduced).

Also, the capacitance element 202B performs a similar operation.

As described above, by providing auxiliary transistors, the sourcevoltage of a voltage relaxing transistor (input side) can be fixed to“Vss” when an input transistor is not driven, so that the breakdownvoltage limit of the voltage relaxing transistor (input side).

Also, by providing capacitance elements, voltage variations at theopposite ends of a voltage relaxing transistor (output side) can becaused to follow each other, thereby making it possible to furtherreduce the source-drain voltage of the voltage relaxing transistor(output side).

Note that, as shown in FIG. 6, when MOS transistors 203A and 203B areprovided instead of the capacitance elements 202A and 202B, a similareffect can be obtained. The gate of the MOS transistor 203A is connectedto the drain of the voltage relaxing transistor 103A, and the source,drain and substrate of the MOS transistor 203A are connected to thesource of the voltage relaxing transistor 103A.

Third Embodiment

FIG. 7 shows a configuration of a level shifter according to a thirdembodiment according to the present invention. This level shiftercomprises, in addition to the parts of FIG. 1, a reset circuit 301 andan inverter circuit 302.

The reset circuit (signal supplying section) 301 receives a referencesignal IN and a reset signal RESET, and when the reset signal RESET isat the “L level”, supplies the reference signal IN as the input signalINP to the gate of the input transistor 101A. The reference signal INand the reset signal RESET are each, for example, a signal having alevel varying between a positive level (Vdd) and a ground level (Vss).On the other hand, when the reset signal RESET is at the “H level”, thereset circuit 301 supplies the “H level” input signal INP to the inputtransistor 101A without depending on the logic level of the referencesignal IN (i.e., the voltage level of the reference signal IN isforcedly fixed to the “H level”).

The inverter circuit (signal inverting section) 302 inverts a signalfrom the reset circuit 301, and supplies the result as the input signalINM to the gate of the input transistor 101B.

Next, a reset operation of the level shifter by the reset circuit 301and the inverter circuit 302 will be described. When the reset signalRESET supplied from the reset circuit 301 goes to the “H level”, asignal (the input signal INP) from the reset circuit 301 goes to the “Hlevel” and a signal (the input signal INM) from the inverter circuit 302goes to the “L level”. Thereby, the voltages (Va3, Vb3) at the outputnodes N11A and N11B can be fixed to the L level (Vssm) and the H level(Vreg), respectively, thereby making it possible to settle the logics ofthe inverter circuits 104A and 104B.

As described above, the level shifter can be forcedly reset, therebymaking it possible to settle the internal states of the inverters 104Aand 104B. Thereby, for example, it is possible to avoid a situation inwhich the output nodes N11A and N11B become stable at an intermediatepotential, so that a through current continues to flow from thereference node N4 to the reference node N5.

Note that the reset circuit 301 and the inverter circuit 302 are alsoapplicable to the level shifters of FIGS. 3, 4 and 6.

1. A level shifter for receiving a pair of input signals varying withcomplementary levels and shifting the levels of the pair of inputsignals, comprising: a pair of input transistors having a pair ofsources connected in common to a first input reference node and a pairof gates to which the pair of input signals are input; a pair ofinput-side voltage relaxing transistors having a pair of sourcesconnected to a pair of drains of the pair of input transistors and apair of gates connected in common to a second input reference node, andfor limiting voltages at the pair of drains of the pair of inputtransistors; a pair of output nodes; a pair of output-side voltagerelaxing transistors having a pair of sources connected to the pair ofoutput nodes, a pair of gates connected in common to a first outputreference node, and a pair of drains connected to the pair drains of thepair of input-side voltage relaxing transistors, and for limitingvoltages at the pair of output nodes; and a first inverter circuit and asecond inverter circuit in one-to-one correspondence with the pair ofoutput nodes and each connected between a second output reference nodeand a third output reference node, wherein each of the first and secondinverter circuits supplies a voltage at one of the second and thirdoutput reference nodes to one corresponding to said inverter circuit ofthe output nodes, depending on a voltage at one not corresponding tosaid inverter circuit of the output nodes.
 2. The level shifter of claim1, wherein each of the first and second inverter circuits includes: afirst select transistor having a source connected to the second outputreference node, a drain connected to the output node corresponding tosaid inverter circuit, and a gate connected to the output node notcorresponding to said inverter circuit; and a second select transistorhaving a source connected to the third output reference node, a drainconnected to the output node corresponding to said inverter circuit, anda gate connected to the output node not corresponding to said invertercircuit.
 3. The level shifter of claim 2, wherein the current drivingability of the pair of input transistors is greater than the currentdriving ability of the second select transistor, the current drivingability of the pair of input-side voltage relaxing transistors isgreater than the current driving ability of the pair of inputtransistors, and the current driving ability of the pair of output-sidevoltage relaxing transistors is greater than the current driving abilityof the second select transistor.
 4. The level shifter of claim 1,wherein a voltage supplied to the second output reference node is equalto a voltage supplied to the first output reference node.
 5. The levelshifter of claim 1, further comprising: a first auxiliary transistorhaving a gate to which one of the pair of input signals is input, asource connected to the second input reference node, and a drainconnected to the source of one of the pair of input-side voltagerelaxing transistors; and a second auxiliary transistor having a gate towhich the other of the pair of input signals is input, a sourceconnected to the second input reference node, and a drain connected tothe source of the other of the pair of input-side voltage relaxingtransistors.
 6. The level shifter of claim 1, further comprising: afirst capacitance section connected between the source and drain of oneof the pair of output-side voltage relaxing transistors; and a secondcapacitance section connected between the source and drain of the otherof the pair of output-side voltage relaxing transistors.
 7. The levelshifter of claim 6, wherein each of the first and second capacitancesections is a capacitance element.
 8. The level shifter of claim 6,wherein each of the first and second capacitance sections is a MOStransistor.
 9. The level shifter of claim 1, further comprising: asignal supplying section for receiving a reference signal and supplyingthe reference signal as one of the pair of input signals to the gate ofone of the pair of input transistors; and a signal inverting section forinverting the reference signal from the signal supplying section andsupplying the inverted reference signal as the other of the pair ofinput signals to the gate of the other of the pair of input transistors.10. The level shifter of claim 9, wherein the signal supplying sectionfurther receives a control signal, and fixes a voltage level of thereference signal, depending on the control signal.